If citadel is trading based on its knowledge of yet to be executed customer order information that is not public then they are committing a crime, and there are no exceptions.Here's how Robinhood is raking in record cash on customer trades — despite making it free
Robinhood is raking in more money than ever on customer trades as new investors flood the stock market during the pandemic.www.cnbc.com
Citadel is not paying Robinhood to execute trades for no reason. I am sure it's structured in a legal way, but the effect is the same, they skim off those free trades and pay Robinhood for the privilege. So when Citadel, which is the source of RH's revenue is short and RH's users who don't pay commissions are long a stock, RH is going to serve its actual paying customer.
Pay for order flow is actually a fairly common thing, but not so people can front run orders.